Piezoelectric device, method for producing piezoelectric device, and liquid discharge device

ABSTRACT

A piezoelectric device of the present invention includes a piezoelectric material and lower and upper electrodes for applying an electric field to the piezoelectric material. The upper electrode is patterned, and an edge portion of the upper electrode is provided with a structure where an intensity of the electric field exerted on the piezoelectric material gradually decreases along a direction from a central portion toward an edge surface of the upper electrode when the electric field is applied to the piezoelectric material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric device, a method forproducing the piezoelectric device, and a liquid discharge device.

2. Description of the Related Art

Piezoelectric devices, which include a piezoelectric material thatexpands or contracts when the intensity of an electric field appliedthereto is increased or decreased and an electrode for applying theelectric field to the piezoelectric material, are used as actuators,etc., provided in liquid discharge devices, such as inkjet recordingheads. As piezoelectric materials, perovskite oxides, such as leadzirconium titanate (PZT), are known.

FIG. 8 shows a basic cross-sectional structure of a piezoelectricdevice. A piezoelectric device 200 includes a substrate 210, and a lowerelectrode 220, a piezoelectric material 230 and an upper electrode 240,which are formed in this order on the substrate 210. In conventional andtypical piezoelectric devices, the lower electrode is formed over theentire surface of the substrate, and the upper electrode is formed in apredetermined pattern. In such a structure, electric charge tends toconcentrate in the piezoelectric material in the vicinity of the edgesurface of the upper electrode, and this may often cause a leakagecurrent at this portion. Further, at the area where the upper electrodeis formed, an electric field is applied to the piezoelectric material tocause displacement. In contrast, at the area where the upper electrodeis not formed, the electric field is not exerted on the piezoelectricmaterial in an active manner, and thus no active displacement occurs atthis area. Therefore, stress also tends to concentrate in thepiezoelectric material in the vicinity of the edge surface of the upperelectrode. Portions where the electric charge and the stress tend toconcentrate are indicated by the circles in the drawing.

Due to the high tendency of the concentration of electric charge andstress, microcracks may be generated in the piezoelectric material inthe vicinity of the edge surface of the upper electrode. When themicrocracks are generated in the surface of the piezoelectric material,moisture enters through these portions to cause deterioration of thepiezoelectric material. In particular, operational durability of thepiezoelectric material in a highly humid environment is lowered. Thisproblem is prominent when the piezoelectric material is in the form of athin film.

Japanese Unexamined Patent Publication No. 2008-147350 (which is hereinafter referred to as Patent Document 1) discloses a production method,wherein the upper electrode has a double-layer structure including afirst upper electrode and a second upper electrode, which are formed inthis order from the piezoelectric material side, the first upperelectrode being formed through a sol-gel method or a MOD process and thesecond upper electrode being formed through a PVD process (claim 1).

Patent Document 1 teaches, in paragraph 0008, for example, thatformation of a low dielectric layer at the upper electrode side of thepiezoelectric material can be prevented, thereby preventing degradationof the displacement property of the piezoelectric material due tovoltage drop caused by the low dielectric layer and deterioration of theleakage property, such as the concentration of electric charge at thelow dielectric layer causing electric breakdown.

Patent Document 1 further teaches that it is preferred to form thepiezoelectric material through the steps of coating a sol of an organicmetal compound to form a piezoelectric material precursor film, andheating and firing the precursor film to form the piezoelectric film(claim 3), and that, when the first upper electrode is formed, it ispreferred to form a first upper electrode precursor film, which will bethe first upper electrode, on the piezoelectric material precursor filmbefore being fired, and to simultaneously fire the piezoelectricmaterial precursor film and the first upper electrode precursor film(claim 4). Patent Document 1 further teaches, in paragraph 0011, forexample, that, according to this method, formation of a different phasebetween the piezoelectric material and the first upper electrode can beprevented, thereby preventing exfoliation due to the different phase andelectric breakdown due to the concentration of electric charge.

The production method disclosed in Patent Document 1 is directed topreventing the exfoliation due to the different phase and theconcentration of electric charge, and is not intended to minimize theconcentration of electric charge in the piezoelectric material in thevicinity of the edge surface of the upper electrode.

Japanese Unexamined Patent Publication No. 2004-207633 (Patent Document2) proposes a stacked piezoelectric device which has an edge portion ofan internal electrode having a smoothly curved sectional shape without acorner (paragraph 0013, FIG. 2, etc.) Patent Document 2 teaches, inparagraph 0013, that this structure can minimize the concentration ofelectric charge at the edge portion of the internal electrode and canmitigate the concentration of stress in the vicinity of the edge portionof internal electrode of the piezoelectric material.

In the technique disclosed in Patent Document 2, the edge portion of theinternal electrode is provided with the rounded shape in order tominimize the concentration of electric charge in the vicinity of theedge portion of the internal electrode of the stacked piezoelectricdevice. The technique does not minimize the concentration of electriccharge in the vicinity of the edge surface of the upper electrode oftypical piezoelectric devices, which are not stacked piezoelectricdevices.

SUMMARY OF THE INVENTION

In view of the above-described circumstances, the present invention isdirected to providing a piezoelectric device with high durability, whichis achieved by mitigating concentration of electric charge and stress inthe piezoelectric material in the vicinity of an edge surface of anupper electrode, and a method for producing the piezoelectric device.

An aspect of the piezoelectric device of the invention includes apiezoelectric material and lower and upper electrodes for applying anelectric field to the piezoelectric material, wherein the upperelectrode is patterned, and an edge portion of the upper electrode isprovided with a structure where an intensity of the electric fieldexerted on the piezoelectric material gradually decreases along adirection from a central portion toward an edge surface of the upperelectrode when the electric field is applied to the piezoelectricmaterial.

In a preferred first aspect of the piezoelectric device of theinvention, a sloped-thickness insulating layer may be formed between thepiezoelectric material and the edge portion of the upper electrode. Thesloped-thickness insulating layer has a thickness that graduallyincreases along the direction from the central area toward the edgesurface of the upper electrode.

The sloped-thickness insulating layer may be formed of a material havingthickness-dependent permittivity.

The sloped-thickness insulating layer may be an organic insulating layermainly composed of polyimide or an inorganic insulating layer mainlycomposed of a Si compound.

The description “mainly composed of” herein means that the content ofthe component(s) is not less than 90% by mass.

The piezoelectric device of the first aspect may be produced by a methodincluding: step (A) of forming an unpatterned insulating layer on asubstrate having the lower electrode and the piezoelectric materialformed thereon; step (B) of forming a resist mask on the insulatinglayer at an area where the upper electrode is not to be formed; and step(C) of partially removing the insulating layer from an area where theupper electrode is to be formed to leave the insulating layer at an edgeportion of the area where the upper electrode is to be formed, the leftinsulating layer having a thickness that gradually increases along thedirection from the central area toward the edge surface of the upperelectrode, thereby forming the sloped-thickness insulating layer.

The step (C) may include partially removing the insulating layer fromthe area where the upper electrode is to be formed through a dry etchingprocess.

In a preferred second aspect of the piezoelectric device of theinvention, the edge portion of the upper electrode may have a gradientcomposition structure where an insulation property of the upperelectrode gradually becomes higher along the direction from the centralarea toward the edge surface of the upper electrode.

In the second aspect, a main portion other than the edge portion of theupper electrode may be mainly composed of a conductive metal, and theedge portion of the upper electrode may have a gradient compositionstructure where a metal oxide content gradually increases along thedirection from the central area toward the edge surface of the upperelectrode.

The piezoelectric device of the second aspect may be produced by amethod including: step (D) of forming the upper electrode having auniform composition mainly composed of a conductive metal on a substratehaving the lower electrode and the piezoelectric material formedthereon; step (E) of forming an oxygen permeating film on the upperelectrode, the oxygen permeating film having such oxygen permeabilitythat an area of the oxygen permeating film corresponding to the mainportion other than the edge portion of the upper electrode has a uniformoxygen transmission rate, and an area of the oxygen permeating filmcorresponding to the edge portion of the upper electrode has an oxygentransmission rate that gradually increases along the direction from thecentral area toward the edge surface of the upper electrode; and step(F) of applying oxidation to the upper electrode covered with the oxygenpermeating film to provide the edge portion of the upper electrode withthe gradient composition structure.

The step (E) may include forming, as the oxygen permeating film, anoxygen permeating film having a sloped-thickness structure where an areacorresponding to the main portion other than the edge portion of theupper electrode has a uniform thickness, and an area corresponding tothe edge portion of the upper electrode has a thickness that graduallydecreases along the direction from the central area toward the edgesurface of the upper electrode.

The oxygen permeating film may be a film having an oxygen permeabilitycoefficient at a temperature of 40° C. of not less than 1.0×10⁻¹¹(cm³(STP)·cm/cm²·s·cmHg).

The oxygen permeability coefficient herein is data at a temperature of40° C., unless otherwise stated.

The piezoelectric device of the second aspect may be produced by amethod including: step (D) of forming the upper electrode having auniform composition mainly composed of a conductive metal on a substratehaving the lower electrode and the piezoelectric material formedthereon; step (G) of forming a covering film on the upper electrode; andstep (H) of applying oxidation to the upper electrode covered with thecovering film from an edge surface side of the upper electrode toprovide the edge portion of the upper electrode with the gradientcomposition structure.

A first aspect of the method for producing a piezoelectric device is amethod for producing the piezoelectric device of the first aspect. Themethod includes: step (A) of forming an unpatterned insulating layer ona substrate having the lower electrode and the piezoelectric materialformed thereon; step (B) of forming a resist mask on the insulatinglayer at an area where the upper electrode is not to be formed; and step(C) of partially removing the insulating layer from an area where theupper electrode is to be formed to leave the insulating layer at an edgeportion of the area where the upper electrode is to be formed, the leftinsulating layer having a thickness that gradually increases along thedirection from the central area toward the edge surface of the upperelectrode, thereby forming the sloped-thickness insulating layer.

A second aspect of the method for producing a piezoelectric device ofthe invention is a method for producing the piezoelectric device of thesecond aspect. The method includes: step (D) of forming the upperelectrode having a uniform composition mainly composed of a conductivemetal on a substrate having the lower electrode and the piezoelectricmaterial formed thereon; step (E) of forming an oxygen permeating filmon the upper electrode, the oxygen permeating film having such oxygenpermeability that an area of the oxygen permeating film corresponding tothe main portion other than the edge portion of the upper electrode hasa uniform oxygen transmission rate, and an area of the oxygen permeatingfilm corresponding to the edge portion of the upper electrode has anoxygen transmission rate that gradually increases along the directionfrom the central area toward the edge surface of the upper electrode;and step (F) of applying oxidation to the upper electrode covered withthe oxygen permeating film to provide the edge portion of the upperelectrode with the gradient composition structure.

A third aspect of the method for producing a piezoelectric device of theinvention is a method for producing the piezoelectric device of thesecond aspect. The method includes: step (D) of forming the upperelectrode having a uniform composition mainly composed of a conductivemetal on a substrate having the lower electrode and the piezoelectricmaterial formed thereon; step (G) of forming a covering film on theupper electrode; and step (H) of applying oxidation to the upperelectrode covered with the covering film from an edge surface side ofthe upper electrode to provide the edge portion of the upper electrodewith the gradient composition structure.

The liquid discharge device of the invention includes: the piezoelectricdevice of the invention; and a liquid discharge member disposed adjacentto the piezoelectric device, the liquid discharge member including aliquid reservoir for storing a liquid, and a liquid discharge port fordischarging the liquid from the liquid reservoir to the outside inresponse to application of the electric field to the piezoelectric film.

According to the invention, a piezoelectric device with high durability,which is achieved by mitigating concentration of electric charge andstress in the piezoelectric material in the vicinity of an edge surfaceof an upper electrode, and a method for producing the piezoelectricdevice can be provided.

According to the invention, a piezoelectric device with good durabilityin a high temperature and high humidity environment at a temperature of40° C. and a relative humidity of 80% and a method for producing thepiezoelectric device can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating the structures of apiezoelectric device and an inkjet recording head according to a firstembodiment of the invention,

FIGS. 2A to 2F illustrates a production process for producing thepiezoelectric device shown in FIG. 1,

FIG. 3 is a sectional view illustrating the structures of apiezoelectric device and an inkjet recording head according to a secondembodiment of the invention,

FIGS. 4A to 4F illustrates a production process for producing thepiezoelectric device shown in FIG. 3,

FIGS. 5A to 5F illustrates another production process for producing thepiezoelectric device shown in FIG. 3,

FIG. 6 is a diagram illustrating a configuration example of an inkjetrecording device including the inkjet recording head shown in FIG. 1 or3,

FIG. 7 is a partial plan view of the inkjet recording device shown inFIG. 6, and

FIG. 8 is a sectional view illustrating a basic structure of aconventional piezoelectric device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment ofPiezoelectric Device and Inkjet Recording Head

The structures of a piezoelectric device and an inkjet recording head(liquid discharge device) including the piezoelectric device accordingto a first embodiment of the invention are described with reference toFIG. 1. FIG. 1 is a sectional view of the main portion of the inkjetrecording head (a sectional view along the thickness direction of thepiezoelectric device). For ease of visual understanding, the componentsshown in the drawing are not to scale.

A piezoelectric device 1 of this embodiment includes a substrate 10, anda lower electrode 20, a piezoelectric material 30 and an upper electrode50, which are formed in this order on the substrate 10. An electricfield in the thickness direction is applied to the piezoelectricmaterial 30 via the lower electrode 20 and the upper electrode 50.

The lower electrode 20 and the piezoelectric material 30 are formed oversubstantially the entire surface of the substrate 10. On thepiezoelectric material 30, the upper electrode 50 is patterned. Thepiezoelectric material 30 may also be patterned. When the piezoelectricmaterial 30 is formed in a pattern including a plurality of separateprotrusions, the individual protrusions can smoothly expand or contract,thereby preferably providing a larger displacement.

In this embodiment, a sloped-thickness insulating layer 40 is formedbetween the piezoelectric material 30 and an edge portion 50E of theupper electrode 50. The sloped-thickness insulating layer 40 has athickness that gradually increases along a direction from the centralarea toward the edge surface of the upper electrode.

The substrate 10 is not particularly limited, and may be any substrate,such as a silicon, silicon oxide, stainless steel (SUS), yttriumstabilized zirconia (YSZ), alumina, sapphire, SiC, or SrTiO₃ substrate.The substrate 10 may be a multilayer substrate, such as a SOI substrateincluding a SiO₂ film and a Si active layer formed in this order on asilicon substrate.

The composition of the lower electrode 20 is not particularly limited,and examples thereof may include a metal or a metal oxide, such as Au,Pt, Ir, IrO₂, RuO₂, LaNiO₃, and SrRuO₃, as well as combinations thereof.The composition of the upper electrodes 50 is not particularly limited,and examples thereof may include the example materials listed for thelower electrode 20, electrode materials commonly used in semiconductorprocesses, such as Al, Ta, Cr and Cu, and combinations thereof. Thethicknesses of the lower electrode 20 and the upper electrodes 50 arenot particularly limited; however, their thicknesses may be in the rangefrom 50 to 500 nm.

A piezoelectric actuator 2 includes a vibrating plate 60, which vibratesalong with expansion and contraction of the piezoelectric material 30,attached on the back side of the substrate 10 of the piezoelectricdevice 1. The piezoelectric actuator 2 also includes a controlling means(not shown), such as a drive circuit, for controlling drive of thepiezoelectric device 1.

An inkjet recording head (liquid discharge device) 3 generally includes,at the back side of the piezoelectric actuator 2, an ink nozzle (liquidstoring and discharging member) 70 including an ink chamber (liquidreservoir) 71 for storing ink and an ink discharge port (liquiddischarge port) 72 through which the ink is discharged from the inkchamber 71 to the outside. In the inkjet recording head 3, thepiezoelectric device 1 expands or contracts when the intensity of theelectric field applied to the piezoelectric device 1 is increased ordecreased, thereby controlling discharge of the ink from the ink chamber71 and the amount of the discharged ink.

Instead of providing the vibrating plate 60 and the ink nozzle 70 whichare members separate from the substrate 10, parts of the substrate 10may be machined to form the vibrating plate 60 and the ink nozzle 70.For example, if the substrate 10 is a multilayer substrate, such as aSOI substrate, the substrate 10 may be etched at the back side thereofto form the ink chamber 71, and then the substrate may be machined toform the vibrating plate 60 and the ink nozzle 70.

The piezoelectric material 30 may take any form, such as a singlecrystal, a bulk ceramic or a film. Considering providing a thinner andsmaller piezoelectric device 1 and productivity, etc., the piezoelectricmaterial 30 may take the form of a film, and may optionally be a thinfilm with a thickness in the range from 10 nm to 100 μm, or furtheroptionally be a thin film with a thickness in the range from 100 nm to20 μm.

The process used to form the piezoelectric material 30 is notparticularly limited, and examples thereof include gas phase processes,such as sputtering, plasma CVD, MOCVD and PLD; liquid phase processes,such as sol-gel method and organic metal decomposition method; andaerosol deposition process.

The composition of the piezoelectric material 30 is not particularlylimited; however, the piezoelectric material 30 may be formed by one ortwo or more perovskite oxides (which may contain inevitable impurities)represented by the formula (P) below:

ABO₃  General Formula (P)

(wherein A represents an A-site element and includes at least oneelement selected from the group consisting of Pb, Ba, Sr, Bi, Li, Na,Ca, Cd, Mg, K and lanthanide elements; B represents a B-site element andincludes at least one element selected from the group consisting of Ti,Zr, V, Nb, Ta, Cr, Mo, W, Mn, Mg, Sc, Co, Cu, In, Sn, Ga, Zn, Cd, Fe,Ni, Hf and Al; O is oxygen; and a molar ratio of A-site element:B-siteelement:oxygen element is 1:1:3 as a standard; however, the molar ratiomay be varied from the standard molar ratio within a range where aperovskite structure is obtained.)

Examples of the perovskite oxides represented by general formula (P)include: lead-containing compounds, such as lead titanate, leadzirconate titanate (PZT), lead zirconate, lead lanthanum titanate, leadlanthanum zirconate titanate, lead magnesium niobate zirconium titanate,lead nickel niobate zirconium titanate and lead zinc niobate zirconiumtitanate, as well as mixed crystal systems thereof; andnon-lead-containing compounds, such as barium titanate, strontium bariumtitanate, bismuth sodium titanate, bismuth potassium titanate, sodiumniobate, potassium niobate and lithium niobate, as well as mixed crystalsystems thereof.

In view of improvement of electrical characteristics, the perovskiteoxide represented by general formula (P) may contain one or two or moremetal ions, such as Mg, Ca, Sr, Ba, Bi, Nb, Ta, W, and Ln (=lanthanideelements: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu).

As described above, in this embodiment, the sloped-thickness insulatinglayer 40, which has a thickness that gradually increases along thedirection from the central area toward the edge surface of the upperelectrode, is formed between the piezoelectric material 30 and the edgeportion 50E of the upper electrode 50. With this structure, a distancebetween the lower electrode 20 and the upper electrode 50 at the areacorresponding to the edge portion 50E of the upper electrode 50gradually increases along the direction from the central area toward theedge surface of the upper electrode. Therefore, at the areacorresponding to the edge portion 50E of the upper electrode 50, when anelectric field is applied to the piezoelectric material 30, intensity ofthe electric field exerted on the piezoelectric material 30 graduallydecreases along the direction from the central area toward the edgesurface of the upper electrode.

The composition of the sloped-thickness insulating layer 40 is notparticularly limited, and the layer may be an organic insulating layeror an inorganic insulating layer. An electric field E1 induced when adielectric material having a relative permittivity ε is inserted betweenelectrodes with an electric field E is expressed by the equation below:

E1=E/ε0

The sloped-thickness insulating layer 40 may be formed of a materialhaving a thickness-dependent permittivity. Some reports have been madeabout change of permittivity of polymeric thin films depending on thefilm thickness, and it is reported that the permittivity exhibits thefilm thickness dependency within the range of the thickness of apolymeric film of not more than about 1.0 μm (“Effect of Film Thicknesson Electrical Properties of Polyimide Thin Films”, T. Liang et al., JSRTECHNICAL REVIEW, No. 109, pp. 6-11, 2002, etc.)

That is, the sloped-thickness insulating layer 40 may be one of variouspolymeric films. Considering that the upper electrode 50 is formed afterthe sloped-thickness insulating layer 40 has been formed, thesloped-thickness insulating layer 40 may be a polymeric film with highheat resistance. Optionally, the sloped-thickness insulating layer 40may be an organic insulating layer mainly composed of polyimide.Further, a maximum film thickness t of the sloped-thickness insulatinglayer 40 may be in the range of not more than 1.0 μm, where thepermittivity of the layer exhibits the film thickness dependency.

Si compounds, such as SiO₂ and Si₃N₄, also have a permittivity thatexhibits the film thickness dependency in the range of the filmthickness of not more than about 1.0 μm. The sloped-thickness insulatinglayer 40 may be an inorganic insulating layer mainly composed of one ortwo or more Si compounds. Also in this case, the maximum film thicknesst of the sloped-thickness insulating layer 40 may be in the range of notmore than 1.0 μm, where the permittivity of the layer exhibits the filmthickness dependency.

With the sloped-thickness insulating layer 40 formed of a materialhaving thickness-dependent permittivity being provided at the areacorresponding to the edge portion 50E of the upper electrode 50, thestructure where the intensity of the electric field exerted on thepiezoelectric material 30 gradually decreases along the direction fromthe central area toward the edge surface of the upper electrode, whenthe electric field is applied to the piezoelectric material 30, cansuccessfully be formed at the area corresponding to the edge portion 50Eof the upper electrode 50.

With the structure of this embodiment, at the area corresponding to theedge portion 50E of the upper electrode 50, the intensity of theelectric field exerted on the piezoelectric material 30 graduallydecreases along the direction from the central area toward the edgesurface of the upper electrode when the electric field is applied to thepiezoelectric material 30, and therefore, the piezoelectric displacementof the piezoelectric material 30 also gradually decreases along thedirection from the central area toward the edge surface of the upperelectrode. Thus, with the structure of this embodiment, theconcentration of electric charge and stress in the piezoelectricmaterial 30 in the vicinity of the edge surface of the upper electrode50 can be mitigated.

An area W2 where the sloped-thickness insulating layer 40 is formed anda slope angle θ are not particularly limited. In this embodiment, theelectric field is applied to the piezoelectric material 30 and thepiezoelectric displacement occurs mainly at the area corresponding to amain portion 50M other than the edge portion 50E of the upper electrode50. If the area W2 is excessively large, satisfactory piezoelectricdisplacement occurs only in a small area. Therefore, the area W2 may bedesigned as small as possible within a range where the effect ofmitigating the concentration of electric charge and stress in thepiezoelectric material 30 in the vicinity of the edge surface of theupper electrode 50 can be provided.

The area W2 may be not more than 5% of a width W1 of the upper electrode50 (i.e., the total width W2 of the sloped-thickness insulating layer 40at the opposite edge portions may be not more than 10% of the width W1).The smaller the slope angle θ of the sloped-thickness insulating layer40, the smaller the gradient of the change of the intensity of theelectric field. The slope angle θ may be not more than 60°, optionallynot more than 45°, or further optionally not more than 30°. With asmaller slope angle θ, a ratio of the area W2 where the sloped-thicknessinsulating layer 40 is formed to the width W1 of the upper electrode isincreased. Therefore, a smaller angel θ may be selected within the rangewhere the ratio of W2/W1 is not more than 5%.

The structures of the piezoelectric device 1 and the inkjet recordinghead 3 of this embodiment are as described above. According to thisembodiment, the concentration of electric charge and stress in thepiezoelectric material 30 in the vicinity of the edge surface of theupper electrode 50 can be mitigated, thereby providing the piezoelectricdevice 1 with high durability. According to this embodiment, thepiezoelectric device 1 with good durability even in a high temperatureand high humidity environment at a temperature of 40° C. and a relativehumidity of 80% can be provided.

Method for Producing Piezoelectric Device of First Embodiment

An example of a method for producing the piezoelectric device of thefirst embodiment is described with reference to the drawings. FIGS. 2Ato 2F illustrate a production process. The substrate 10 and the lowerelectrode 20 are not shown in these drawings.

First, the lower electrode 20 and the piezoelectric material 30 areformed on the substrate 10 using a conventional process (not shown).Then, as shown in FIG. 2A, a film of a material of the sloped-thicknessinsulating layer 40 is formed on the piezoelectric material 30 to forman unpatterned insulating layer 40A (step (A)).

If the insulating layer 40A is formed of an organic material, theinsulating layer 40A may be formed, for example, through a processincluding preparing a solution containing the organic material dissolvedin a solvent, coating the solution on the piezoelectric material 30using any of various coating method, such as spin coating and dipcoating, or one of various printing processes, such as screen printing,and drying the coated solution to remove the solvent. If the insulatinglayer 40A is a polyimide film, the film may be formed using a polyamicacid, which is the precursor, and then removal of the solvent andimidization may be achieved through heating.

If the insulating layer 40A is formed of an organic material, examplesof the process used to form the insulating layer 40A may include gasphase processes, such as sputtering, CVD, and vapor deposition; andchemical liquid phase processes, such as sol-gel method and MOD (metalorganic deposition).

Then, as shown in FIG. 2B, a resist mask RM is formed at an area A2,where the upper electrode 50 is not to be formed, on the insulatinglayer 40A (step (B)). In the drawing, the symbol A1indicates an areawhere the upper electrode 50 is to be formed.

Then, as shown in FIG. 2C, the insulating layer 40A is partially removedfrom the area A1 where the upper electrode 50 is to be formed, so thatthe insulating layer having a thickness that gradually increases alongthe direction from the central area toward the edge surface of the upperelectrode is left to form the sloped-thickness insulating layer 40 atthe edge portion of the area A1 (step (C)). In the drawings, the symbol40B indicates the insulating layer after step (C).

Examples of the process used to partially remove the insulating layer40A from the area A1 where the upper electrode 50 is to be formed mayinclude dry etching and wet etching, and in particular, dry etching maybe used. The area W2 where the sloped-thickness insulating layer 40 isformed and the slope angle θ can be controlled by controlling thethickness of the resist mask RM and etching conditions.

Although the dry etching is known as an anisotropic etching process, thearea W2 where the sloped-thickness insulating layer 40 is formed and theslope angle θ can be controlled by forming the resist mask RM having arelatively large thickness and controlling collision of gas moleculesagainst the insulating layer 40A to be close to that of isotropicetching with utilizing the shadow of the resist mask RM. Although thethickness of the resist mask in conventional dry etching processes is onthe order of several micrometers, the thickness of the resist mask RM inthis embodiment may, for example, be about 3 to 10 μm.

Then, as shown in FIG. 2D, a film of a material of the upper electrode50 is formed on the substrate 10, which has the insulating layer 40Bformed thereon, to form an unpatterned upper electrode 50A. Then, asshown in FIG. 2E, the resist mask RM and the upper electrode 50A formedon the resist mask RM are removed through a liftoff process.

Finally, as shown in FIG. 2F, the insulating layer 40B at the area A2where the upper electrode 50 is not formed is removed through etching.In this manner, piezoelectric device 1 of the above-described embodimentis produced. The step shown in FIG. 2F may not be carried out sincethere is no problem if the insulating layer 40B is left at the area A2where the upper electrode 50 is not formed. It should be noted that themethod for producing the piezoelectric device of the first embodiment isnot limited to the above-described process, and may be modified asappropriate.

Second Embodiment of Piezoelectric Device and Inkjet Recording Head

The structures of a piezoelectric device and an inkjet recording head(liquid discharge device) including the piezoelectric device accordingto a second embodiment of the invention are described with reference toFIG. 3. FIG. 3 is a sectional view illustrating the main portion of theinkjet recording head (a sectional view taken along the thicknessdirection of the piezoelectric device). For ease of visualunderstanding, the components shown in the drawing are not to scale.Components that are the same as those in the first embodiment aredenoted by the same reference symbols, and explanation thereof isomitted.

A piezoelectric device 4 of this embodiment includes the substrate 10,and the lower electrode 20, the piezoelectric material 30 and an upperelectrode 80 which are sequentially formed on the substrate 10. Anelectric field in the thickness direction is applied to thepiezoelectric material 30 via the lower electrode 20 and the upperelectrode 80. Similarly to the first embodiment, the lower electrode 20and the piezoelectric material 30 are formed over substantially theentire surface of the substrate 10, and the upper electrode 80 ispatterned on the piezoelectric material 30.

In this embodiment, the sloped-thickness insulating layer 40 is notprovided, and an edge portion 80E of the upper electrode 80 has agradient composition structure, where the insulation property of theupper electrode 80 gradually becomes higher along the direction from thecentral area toward the edge surface of the upper electrode.

The gradient composition structure may be such that a main portion 80Mother than the edge portion 80E of the upper electrode 80 mainlycomposed of a conductive metal, whereas the edge portion 80E of theupper electrode 80 has the gradient composition structure where a metaloxide content gradually increases along the direction from the centralarea toward the edge surface of the upper electrode.

In view of ease of providing an oxide, the main component of the mainportion 80M of the upper electrode 80 may be one or tow or moreconductive metals, such as Ir, Al, Ta, Cr, Ti, Zn, Sn and Cu. Thethickness of the upper electrode 80 is not particularly limited;however, the thickness may be in the range from 50 to 500 nm.

A piezoelectric actuator 5 includes the vibrating plate 60, which isattached on the back side of the substrate 10 of the piezoelectricdevice 4. An inkjet recording head (liquid discharge device) 6 includesthe ink nozzle (liquid storing and discharging member) 70, which isattached on the back side of the piezoelectric actuator 5.

In this embodiment, the edge portion 80E of the upper electrode 80 hasthe gradient composition structure where the insulation property of theupper electrode 80 gradually becomes higher along the direction from thecentral area toward the edge surface of the upper electrode. Therefore,at the area corresponding to the edge portion 80E of the upper electrode80, the intensity of the electric field exerted on the piezoelectricmaterial 30 gradually decreases along the direction from the centralarea toward the edge surface of the upper electrode when the electricfield is applied to the piezoelectric material 30, and therefore, thepiezoelectric displacement of the piezoelectric material 30 graduallydecreases along the direction from the central area toward the edgesurface of the upper electrode. Thus, with the structure of thisembodiment, the concentration of electric charge and stress in thepiezoelectric material 30 in the vicinity of the edge surface of theupper electrode 80 can be mitigated.

An area W3 where the gradient composition structure is formed is notparticularly limited. In this embodiment, the electric field is appliedto the piezoelectric material 30 and the piezoelectric displacementoccurs mainly at the area corresponding to the main portion 80M of theupper electrode 80 other than the edge portion 80E. If the area W3 isexcessively large, satisfactory piezoelectric displacement occurs onlyin a small area. Therefore, the area W3 may be designed as small aspossible within a range where the effect of mitigating the concentrationof electric charge and stress in the piezoelectric material 30 in thevicinity of the edge surface of the upper electrode 80 can be provided.The area W3 may be not more than 5% of a width W1 of the upper electrode80 (i.e., the total width W3 of the gradient composition structure atthe opposite edge portions may be not more than 10% of the width W1).

The structures of the piezoelectric device 4 and the inkjet recordinghead 6 of this embodiment are as described above. According to thisembodiment, the concentration of electric charge and stress in thepiezoelectric material 30 in the vicinity of the edge surface of theupper electrode 80 can be mitigated, thereby providing the piezoelectricdevice 4 with high durability. According to this embodiment, thepiezoelectric device 4 with good durability in a high temperature andhigh humidity environment at a temperature of 40° C. and a relativehumidity of 80% can be provided.

Method for Producing Piezoelectric Device of Second Embodiment

An example of a method for producing the piezoelectric device of thesecond embodiment is described with reference to the drawings.

FIGS. 4A to 4F illustrate a production process. The substrate 10 and thelower electrode 20 are not shown in these drawings.

First, the lower electrode 20 and the piezoelectric material 30 areformed on the substrate 10 using a conventional process (not shown).Then, as shown in FIG. 4A, a film of a material of the main portion 80Mof the upper electrode 80 is formed on the piezoelectric material 30 toform an unpatterned upper electrode 80A having a uniform compositionwhich is mainly composed of a conductive metal. The process used to formthe upper electrode 80A is not particularly limited, and examplesthereof may include gas phase processes, such as sputtering, CVD, andvapor deposition; and chemical liquid phase processes, such as sol-gelmethod and MOD (metal organic deposition).

Then, as shown in FIG. 4B, the resist mask RM is formed at the area A1where the upper electrode 80 is to be formed. Then, as shown in FIG. 4C,patterning of the upper electrode 80A and removal of the resist mask RMare carried out to form a patterned upper electrode 80B having theuniform composition and mainly composed of the conductive metal (step(D)).

Then, as shown in FIG. 4D, an oxygen permeating film 90 is formed on theupper electrode 80B (step (E)). The oxygen permeating film 90 has suchoxygen permeability that an area of the oxygen permeating filmcorresponding to the main portion 80M of the upper electrode 80 has auniform oxygen transmission rate, and an area of the oxygen permeatingfilm corresponding to the edge portion 80E of the upper electrode 80 hasan oxygen transmission rate that gradually increases along the directionfrom the central area toward the edge surface of the upper electrode.

The oxygen permeating film 90 having the above-described oxygenpermeability may be an oxygen permeating film having a sloped-thicknessstructure where the area corresponding to the main portion 80M of theupper electrode 80 has a uniform thickness and the area corresponding tothe edge portion 80E of the upper electrode 80 has a thickness thatgradually decreases along the direction from the central area toward theedge surface of the upper electrode. In the drawing, the symbol 90Mindicates the main portion of the oxygen permeating film 90 having auniform thickness, and the symbol 90E indicates the edge portion of theoxygen permeating film 90 having the sloped-thickness structure wherethe thickness gradually decreases along the direction from the centralarea toward the edge surface of the upper electrode.

The oxygen permeating film having the sloped-thickness structure can beformed by, first, forming an oxygen permeating film having a uniformthickness, and then, partially removing the formed film. Examples of theprocess used to partially remove the oxygen permeating film may includedry etching and wet etching, and in particular, dry etching may be used.The area where the sloped-thickness structure is formed and the slopeangle can be controlled by controlling the thickness of the resist maskRM and etching conditions.

Although the dry etching is known as an anisotropic etching process, thearea where the sloped-thickness structure is formed and the slope anglecan be controlled by forming the resist mask having a relatively largethickness and controlling collision of gas molecules against the oxygenpermeating film to be close to that of isotropic etching with utilizingthe shadow of the resist mask. Although the thickness of the resist maskin conventional dry etching processes is on the order of severalmicrometers, the thickness of the resist mask in this embodiment may,for example, be about 3 to 10 μm.

The composition of the oxygen permeating film 90 is not particularlylimited, and the oxygen permeating film 90 may be any polymeric film oran inorganic film, such as a gas-permeable porous ceramic film. Any ofpolymeric films is oxygen permeable due to the free volume of polymers.

In view of ease of oxidation of the edge portion of the upper electrode80B in the subsequent step, the oxygen permeating film 90 may have anoxygen permeability coefficient at 40° C. of not less than 1.0×10⁻¹¹(cm³(STP)·cm/cm²·s·cmHg). Examples of the oxygen permeating film 90having the oxygen permeability in this range may include polyethylenefilm (for example, 1.5×10⁻¹⁰ (cm³(STP)·cm/cm²·s·cmHg), silicone film(for example, 5.0×10⁻⁸ (cm³(STP)·cm/cm²·s·cmHg) and siliconepolycarbonate film (for example, 7.5×10⁻¹¹ (cm³(STP)·cm/cm²·s·cmHg). Thedata of the oxygen permeability coefficients here is found in “PolymerHandbook”.

The maximum film thickness of the oxygen permeating film 90 is notparticularly limited, and is designed as appropriate depending on theoxygen permeability coefficient. The maximum film thickness of theoxygen permeating film 90 may, for example, in the range from about 100to 500 μm.

If the oxygen permeating film 90 is a polymeric film, the oxygenpermeating film 90 may be formed, for example, through a processincluding preparing a solution containing the polymer dissolved in asolvent, coating the solution on the upper electrode 80B using one ofvarious printing processes, such as screen printing, and drying thecoated solution to remove the solvent. A polymer precursor may be usedto form the coated film.

If the oxygen permeating film 90 is an inorganic film, examples of theprocess used to form the oxygen permeating film 90 may include gas phaseprocesses, such as sputtering, CVD, and vapor deposition; and chemicalliquid phase processes, such as sol-gel method and MOD (metal organicdeposition).

Then, as shown in FIG. 4E, the upper electrode 80B covered with theoxygen permeating film 90 is oxidized to provide the edge portion 80E ofthe upper electrode 80B with the gradient composition structure (step(F)).

The oxidation may be achieved by heating under the presence of oxygen,such as oxygen or air, for example. The heating temperature and theoxidation time are not particularly limited, and may be designed asappropriate within a range where sufficient oxidation of the edgeportion 80E of the upper electrode 80B is achieved, depending on theoxygen concentration in the atmosphere as well as the oxygenpermeability coefficient and the film thickness of the oxygen permeatingfilm 90. The heating temperature may, for example, be in the range from50 to 100° C. The oxidation time may be in the range from 1 to 24 hours.

Finally, as shown in FIG. 4F, the oxygen permeating film 90 is removed.In this manner, the piezoelectric device 4 of the above-describedembodiment is produced. If the organic oxygen permeating film 90 is anorganic film, the removal of the organic oxygen permeating film 90 maybe achieved by dissolving the film with a solvent. If the organic oxygenpermeating film 90 is an inorganic film, the inorganic oxygen permeatingfilm 90 may be removed through dry etching or wet etching.

Another Method for Producing Piezoelectric Device of Second Embodiment

An example of another method for producing the piezoelectric device ofthe second embodiment is described with reference to the drawings. FIGS.5A to 5F illustrate a production process. The substrate 10 and the lowerelectrode 20 are not shown in these drawings.

First, the lower electrode 20 and the piezoelectric material 30 areformed on the substrate 10 using a conventional process (not shown).Then, as shown in FIG. 5A, a film of a material of the main portion 80Mof the upper electrode 80 is formed on the piezoelectric material 30 toform an unpatterned upper electrode 80A having a uniform compositionwhich is mainly composed of a conductive metal.

Then, as shown in FIG. 5B, the resist mask RM is formed at the area A1where the upper electrode 80 is to be formed. Then, as shown in FIG. 5C,patterning of the upper electrode 80A and removal of the resist mask RMare carried out to form a patterned upper electrode 80B having theuniform composition and mainly composed of the conductive metal (step(D)).

Then, as shown in FIG. 5D, a covering film 91 is formed on the upperelectrode 80B (step (G)). In this embodiment, the upper electrode 80B isoxidized in the subsequent step only from the edge surface side thereof.The covering film 91 is provided to hinder oxidation of the upperelectrode 80B from the top surface side thereof.

The composition of the covering film 91 is not particularly limited, andthe covering film 91 may be any organic film, such as a polymeric film,or any inorganic film. Since the covering film 91 serves to hinderoxidation of the upper electrode 80B from the top surface side thereof,the covering film 91 preferably has a low oxygen permeabilitycoefficient. The covering film 91 may have an oxygen permeabilitycoefficient at 40° C. of less than 1.0×10⁻¹¹ (cm³(STP)·cm/cm²·s·cmHg).The film thickness of the covering film 91 is not particularly limited,and may be designed as appropriate within a range where oxidation of theupper electrode 80B from the top surface side thereof can sufficientlybe hindered, depending on the oxygen permeability coefficient of thefilm.

If the covering film 91 is a polymeric film, the covering film 91 may beformed, for example, through a process including preparing a solutioncontaining the polymer dissolved in a solvent, coating the solution onthe upper electrode 80B using one of various printing processes, such asscreen printing, and drying the coated solution to remove the solvent. Apolymer precursor may be used to form the coated film.

If the covering film 91 is an inorganic film, examples of the processused to form the covering film 91 may include gas phase processes, suchas sputtering, CVD, and vapor deposition; and chemical liquid phaseprocesses, such as sol-gel method and MOD (metal organic deposition).

Then, as shown in FIG. 5E, the upper electrode 80B covered with thecovering film 91 is oxidized from the edge surface side thereof toprovide the edge portion 80E of the upper electrode 80B with thegradient composition structure (step (H)). The oxidation may be achievedby heating under the presence of oxygen, such as oxygen or air, orthrough an oxygen plasma ashing process, for example.

Finally, as shown in FIG. 5F, the covering film 91 is removed. In thismanner, the piezoelectric device 4 of the above-described embodiment isproduced. The removal of the covering film 91 is achieved in the samemanner as is described for the oxygen permeating film 90 shown in FIGS.4A to 4F.

It should be noted that the method for producing the piezoelectricdevice of the second embodiment is not limited to the processes shown inFIGS. 4A to 4F and FIGS. 5A to 5F, and may be modified as appropriate.

Inkjet Recording Device

Now, an example configuration of an inkjet recording device includingthe inkjet recording head 3 or 6 of the above-described embodiments isdescribed with reference to FIGS. 6 and 7. FIG. 6 shows the entiredevice configuration, and FIG. 7 is a partial plan view of the device.

An inkjet recording device 100 shown in the drawings generally includes:a printing section 102 having a plurality of inkjet recording heads(hereinafter simply referred to as “heads”) 102K, 102C, 102M and 102Yprovided correspondingly to ink colors; an ink storing and chargingsection 114 for storing inks to be fed to the heads 102K, 102C, 102M and102Y; a paper feeding section 118 for feeding recording paper 116; adecurling section 120 for decurling the recording paper 116; a suctionbelt conveyer section 122 disposed to face to the nozzle surface (inkdischarge surface) of the printing section 102, for conveying therecording paper 116 with keeping the flatness of the recording paper116; a print detection section 124 for reading the result of printing atthe printing section 102; and a paper discharge section 126 fordischarging the printed recording paper (a print) to the outside.

Each of the heads 102K, 102C, 102M and 102Y forming the printing section102 corresponds to the inkjet recording head 3 or 6 of theabove-described embodiments.

At the decurling section 120, the recording paper 116 is decurled by aheating drum 130 heating the recording paper 116 in a direction oppositeto the direction of the curl.

In the device using the roll paper, a cutter 128 is provided downstreamthe decurling section 120, as shown in FIG. 6, so that the roll paper iscut by the cutter into a sheet of a desired size. The cutter 128 isformed by a fixed blade 128A, which has a length equal to or larger thanthe width of the conveyance path for the recording paper 116, and around blade 128B, which moves along the fixed blade 128A. The fixedblade 128A is disposed on the back surface side of the print, and theround blade 128B is disposed on the print surface side via theconveyance path. In a case where the device uses cut sheets, the cutter128 is not necessary.

The decurled and cut recording paper sheet 116 is sent to the suctionbelt conveyer section 122. The suction belt conveyer section 122includes an endless belt 133 wrapped around rollers 131 and 132, and isadapted such that at least an area of the belt facing the nozzle surfaceof the printing section 102 and a sensor surface of the print detectionsection 124 forms a horizontal (flat) surface.

The belt 133 has a width that is larger than the width of the recordingpaper sheet 116, and a number of suction holes (not shown) are formed inthe belt surface. A suction chamber 134 is provided on the inner side ofthe belt 133 wrapped around the rollers 131 and 132 at a position wherethe suction chamber 134 faces to the nozzle surface of the printingsection 102 and the sensor surface of the print detection section 124. Asuction force generated by a fan 135 provides the suction chamber 134with a negative pressure, thereby holding the recording paper sheet 116on the belt 133 with suction.

As a motive force from a motor (not shown) is transmitted to at leastone of the rollers 131 and 132, around which the belt 133 is wrapped,the belt 133 is driven in the clockwise direction in FIG. 6, and therecording paper sheet 116 held on the belt 133 is conveyed from the leftto the right in FIG. 6.

In a case where margin-less printing, or the like, is carried out, theinks adhere on the belt 133. Therefore, a belt cleaning section 136 isprovided at a predetermined position (any appropriate position otherthan the print region) on the outer side of the belt 133.

A heating fan 140 is provided upstream the printing section 102 alongthe paper sheet conveyance path formed by the suction belt conveyersection 122. The heating fan 140 blows heating air onto the recordingpaper sheet 116 to heat the recording paper sheet 116 before printing.Heating the recording paper sheet 116 immediately before printingpromotes drying of the deposited ink.

The printing section 102 is a so-called full-line head, in which lineheads, each having a length corresponding to the maximum paper width,are arranged in a direction (main scanning direction) perpendicular tothe paper feed direction (see FIG. 7). Each recording head 102K, 102C,102M, 102Y is formed by a line head, which has a plurality of inkdischarge ports (nozzles) provided across a length that is larger thanat least one side of the recording paper sheet 116 of the maximum sizeprintable by the inkjet recording device 100.

The heads 102K, 102C, 102M and 102Y respectively corresponding to thecolor inks of black (K), cyan (C), magenta (M) and yellow (Y) aredisposed in this order from the upstream side along the feed directionof the recording paper sheet 116. By discharging the color inks from theheads 102K, 102C, 102M and 102Y while the recording paper sheet 116 isconveyed, a color image is recorded on the recording paper sheet 116.

The print detection section 124 is formed by a line sensor, or the like,which images the result of ink droplets deposited by the printingsection 102, and the image of the deposited ink droplets read by theline sensor is used to detect discharge defects, such as clogging of thenozzles.

A drying section 142 formed, for example, by a heating fan for dryingthe printed image surface is disposed downstream the print detectionsection 124. Since contact with the printed surface should be avoideduntil the printed inks dry, blowing hot air may be preferred.

A heating and pressurizing section 144 for controlling the gloss of theimage surface is disposed downstream the drying section 142. At theheating and pressurizing section 144, the image surface is pressed witha pressure roller 145 having a predetermined textured pattern on thesurface thereof while the image surface is heated, thereby transferringthe textured pattern onto the image surface.

The thus obtained print is discharged at the paper discharge section126. Prints of intended images (prints on which intended images areprinted) and test prints may separately be discharged. The inkjetrecording device 100 includes a sorting means (not shown) for sortingthe prints of intended images and the test prints and switching thedischarge paths to selectively send the prints of intended images andthe test prints to a discharge section 126A or 126B.

In a case where an intended image and a test print are printed at thesame time on a larger paper sheet, a cutter 148 may be provided to cutoff the test print area.

The configuration of the inkjet recording device 100 is as describedabove.

Modifications

The invention is not limited to the above-described embodiments, and maybe modified as appropriate without departing from the spirit and scopeof the invention.

EXAMPLES

Now, examples according to the invention and a comparative example aredescribed.

Example 1

A piezoelectric device of the invention was provided in the followingmanner according to the process shown in FIGS. 2A to 2F.

As a substrate for film formation, a substrate with an electrode wasprepared, which included a 30 nm-thick Ti adhesion layer and a 150nm-thick Pt lower electrode formed on a Si wafer in this order. Then, a5 μm-thick PZT piezoelectric film was formed using aPb_(1.3)Zr_(0.52)Ti_(0.48)O₃ target and a RF sputtering apparatus. Filmformation conditions were as follows:

-   -   substrate temperature: 525° C.,    -   voltage applied to target: 2.5 W/cm²,    -   substrate-target distance: 60 mm,    -   degree of vacuum: 0.5 Pa, and    -   film formation gas: Ar/O₂ mixed gas (O₂ partial pressure was 1.3        mol %).

Then, a solution containing a polyimide dissolved in a solvent wascoated on the piezoelectric film by spin coating, and the coatedsolution was subjected to a heat treatment at 300° C. for one hour toremove the solvent, thereby forming an unpatterned polyimide insulatinglayer. The film thickness was 200 nm.

Then, an 8 μm-thick resist mask was formed at an area on the insulatinglayer where the upper electrode is not formed, and the insulating layerwas subjected to dry etching. Etching conditions were as follows:

-   -   Ar/CHF₃/CF₄ plasma etching,    -   pressure: 130 Pa,    -   Ar flow rate: 800 sccm,    -   CHF₃ flow rate: 60 sccm,    -   CF₄ flow rate: 60 sccm,    -   applied radio frequency wave: 400 kHz,    -   radio frequency power density: 3 W/cm².

After the dry etching, an insulating layer having a thickness thatgradually increases along the direction from the central area toward theedge surface of the upper electrode was left at an edge portion of thearea where the upper electrode was to be formed, thereby forming thesloped-thickness insulating layer. The area W2 where thesloped-thickness insulating layer was formed was 5% of the width W1 ofthe upper electrode (the total width W2 of the sloped-thicknessinsulating layer at the opposite edges was 10% of the width W1), and theslope angle θ was 60°. The slope angle θ can be controlled bycontrolling the conditions, such as the gas flow rate and the pressure.For example, by decreasing the gas flow rate and/or increasing thepressure, a smaller slope angle θ can be provided.

Thereafter, the steps shown in FIGS. 2D to 2F were carried out toprovide the piezoelectric device of the invention. The upper electrodehad a multi-layer structure including a 50 nm-thick Ti layer and a 200nm-thick Pt layer. The pattern of the upper electrode was a circularpattern with a diameter of 1000 μm.

Since it is difficult to directly measure that the intensity of theelectric field exerted on the edge portion of the upper electrodegradually decreases along the direction from the central area toward theedge surface of the upper electrode, a distribution of temperature ofheat emitted by the piezoelectric material, which is proportional to theintensity of the electric field, was evaluated to achieve indirectverification. The temperature was measured using a thermo microscope,and a two-dimensional temperature distribution on the surface of theupper electrode under drive conditions of drive voltage of 30V and drivefrequency of 100 kHz was evaluated. Since there is no thermal radiationon a metal electrode, a black spray with an emissivity of 0.95 wassprayed over the entire surface of the upper electrode, and atemperature distribution over the entire surface of the upper electrodewas measured. As a result, the temperature was 55° C. at the mainportion of the upper electrode, whereas, at the edge portion, thetemperature gradually decreased along the direction from the centralarea toward the edge surface of the upper electrode, and the temperaturewas 25° C. at the edge of the upper electrode. Through theabove-described test, it was indirectly confirmed that the graduallydecreasing electric field was provided at the edge portion of the upperelectrode, as intended.

To the resulting piezoelectric device, a trapezoidal wave was appliedunder conditions of applied voltage of 50 kV/cm and frequency of 10 kHzin an environment of temperature of 40° C. and relative humidity of 85%,and the number of drive cycles when a dielectric dissipation factorreached 20% was measured to determine a durability life of thepiezoelectric device. The durability life was about 30 billion cycles,which was higher than a target durability for practical use of 10billion cycles.

Example 2

A piezoelectric device of the invention was provided in the same manneras in Example 1, except that a SiO₂ film was formed as thesloped-thickness insulating layer. The durability life was measured inthe same manner as in Example 1, and was found to be about 30 billioncycles, which was higher than the target durability for practical use of10 billion cycles.

Example 3

A piezoelectric device of the invention was provided in the followingmanner according to the process shown in FIGS. 4A to 4F.

A 30 nm-thick Ti adhesion layer, a 150 nm-thick Pt lower electrode and a5 μm-thick PZT piezoelectric film were formed in this order on a Siwafer in the same manner as in Example 1. Then, a Cr film was formed onthe piezoelectric film to form a 300 nm-thick unpatterned upperelectrode having a uniform composition, and then patterning of the upperelectrode was carried out using a resist mask.

Then, on the upper electrode, a polyethylene oxygen permeating film(oxygen permeability coefficient=1.5×10⁻¹⁰ (cm³(STP)·cm/cm²·s·cmHg))having the sloped-thickness structure, where the area corresponding tothe main portion of the upper electrode has a uniform thickness and thearea corresponding to the edge portion of the upper electrode has athickness that gradually decreases along the direction from the centralarea toward the edge surface of the upper electrode, was formed. Themaximum film thickness of the polyethylene oxygen permeating film was 50μm.

Then, the upper electrode covered with the oxygen permeating film wassubjected to oxidation. The oxidation was achieved through a heattreatment at 50° C. under oxygen flow atmosphere. With this treatment,the edge portion of the upper electrode was provided with the gradientcomposition structure where a metal oxide content gradually increasedalong the direction from the central area toward the edge surface of theupper electrode.

In order to check whether the intended composition gradient was providedat the edge portion of the upper electrode, quantification analysis ofoxidation degree was conducted through EDX (energy dispersive X-raymicroanalysis). At the edge portion of the upper electrode, a peakintensity indicating CrO_(x) (chromium oxide) gradually increased from 0to 1 along the direction from the central area toward the edge surfaceof the upper electrode, and thus it was confirmed that the oxidationdegree gradually increased. The area W3 where the gradient compositionstructure was formed was 5% of the width W1 of the upper electrode (thetotal width W3 of the gradient composition structure at the oppositeedges was 10% of W1).

Finally, the oxygen permeating film was removed by being dissolved witha solvent, thereby providing the piezoelectric device of the invention.

Using a thermo microscope, whether the intensity of the electric fieldexerted on the piezoelectric material at the edge portion of the upperelectrode gradually decreased along the direction from the central areatoward the edge surface of the upper electrode was indirectly evaluatedin the same manner as in Example 1. The temperature distribution wasmeasured in the same manner as in Example 1 under drive conditions ofdrive voltage of 30 V and drive frequency of 100 kHz. The temperaturewas 55° C. at the main portion of the upper electrode, whereas, at theedge portion, the temperature gradually decreased along the directionfrom the central area toward the edge surface of the upper electrode,and the temperature was 25° C. at the edge of the upper electrode.

The durability life was measured in the same manner as in Example 1, andwas found to be about 30 billion cycles, which was higher than thetarget durability for practical use of 10 billion cycles.

Example 4

A piezoelectric device of the invention was provided in the same manneras in Example 3, except that the upper electrode was an Ir electrode.Similarly to Example 3, the formed upper electrode had the gradientcomposition structure. The durability life was measured in the samemanner as in Example 1, and was found to be about 30 billion cycles,which was higher than the target durability for practical use of 10billion cycles.

Example 5

A piezoelectric device of the invention was provided in the followingmanner according to the process shown in FIGS. 5A to 5F.

A 30 nm-thick Ti adhesion layer, a 150 nm-thick Pt lower electrode and a5 μm-thick PZT piezoelectric film were formed in this order on a Siwafer in the same manner as in Example 1. Then, a Cr film was formed onthe piezoelectric film to form a 300 nm-thick unpatterned upperelectrode having a uniform composition, and then patterning of the upperelectrode was carried out using a resist mask.

Then, on the upper electrode, a polyacrylonitrile film, which had auniform thickness and low oxygen permeability, i.e., high oxygen barrierfunction (film thickness was 1 μm, oxygen permeability coefficient was8×10⁻¹⁶ (cm³(STP)·cm/cm²·s·cmHg)), was formed as the covering film.

Then, the upper electrode covered with the covering film was subjectedto oxidation. The oxidation was achieved through a heat treatment at 50°C. under oxygen flow atmosphere. With this treatment, the edge portionof the upper electrode was provided with the gradient compositionstructure where a metal oxide content gradually increased along thedirection from the central area toward the edge surface of the upperelectrode.

In order to check whether the intended composition gradient was providedat the edge portion of the upper electrode, quantification analysis ofoxidation degree was conducted through EDX (energy dispersive X-raymicroanalysis), in the same manner as in Example 3. At the edge portionof the upper electrode, a peak intensity indicating CrO_(x) (chromiumoxide) gradually increased from 0 to 1 along the direction from thecentral area toward the edge surface of the upper electrode, and thus itwas confirmed that the oxidation degree gradually increased. The area W3where the gradient composition structure was formed was 5% of the widthW1 of the upper electrode (the total width W3 of the gradientcomposition structure at the opposite edges was 10% of W1).

Finally, the covering film was removed by being dissolved with asolvent, thereby providing the piezoelectric device of the invention.

Using a thermo microscope, whether the intensity of the electric fieldexerted on the piezoelectric material at the edge portion of the upperelectrode gradually decreased along the direction from the central areatoward the edge surface of the upper electrode was indirectly evaluatedin the same manner as in Example 1. The temperature distribution wasmeasured in the same manner as in Example 1 under drive conditions ofdrive voltage of 30 V and drive frequency of 100 kHz. The temperaturewas 55° C. at the main portion of the upper electrode, whereas, at theedge portion, the temperature gradually decreased along the directionfrom the central area toward the edge surface of the upper electrode,and the temperature was 25° C. at the edge of the upper electrode.

The durability life was measured in the same manner as in Example 1, andwas found to be about 30 billion cycles, which was higher than thetarget durability for practical use of 10 billion cycles.

Comparative Example 1

A 30 nm-thick Ti adhesion layer, a 150 nm-thick Pt lower electrode and a5 μm-thick PZT piezoelectric film were formed in this order on a Siwafer in the same manner as in Example 1. Thereafter, the upperelectrode was patterned through a conventional photolithography processto provide a piezoelectric device of a comparative example. The upperelectrode had a multi-layer structure including a 50 nm-thick Ti layerand a 200 nm-thick Pt layer. The pattern of the upper electrode was acircular pattern with a diameter of 1000 μm.

The durability life was measured in the same manner as in Example 1, andwas found to be about 5 billion cycles, which was lower than the targetdurability for practical use of 10 billion cycles.

INDUSTRIAL APPLICABILITY

The piezoelectric device and the method for producing the piezoelectricdevice of the invention are preferably applicable to piezoelectricactuators provided in inkjet recording heads, magnetic read/write heads,MEMS (Micro Electro-Mechanical Systems) devices, micropumps, ultrasoundprobes, ultrasound motors, etc., and ferroelectric devices, such asferroelectric memory.

1. A piezoelectric device comprising a piezoelectric material and lowerand upper electrodes for applying an electric field to the piezoelectricmaterial, wherein the upper electrode is patterned, and an edge portionof the upper electrode comprises a structure where an intensity of theelectric field exerted on the piezoelectric material gradually decreasesalong a direction from a central portion toward an edge surface of theupper electrode when the electric field is applied to the piezoelectricmaterial.
 2. The piezoelectric device as claimed in claim 1 furthercomprising a sloped-thickness insulating layer formed between thepiezoelectric material and the edge portion of the upper electrode, thesloped-thickness insulating layer having a thickness that graduallyincreases along the direction from the central area toward the edgesurface of the upper electrode.
 3. The piezoelectric device as claimedin claim 2, wherein the sloped-thickness insulating layer comprises amaterial having thickness-dependent permittivity.
 4. The piezoelectricdevice as claimed in claim 3, wherein the sloped-thickness insulatinglayer comprises an organic insulating layer mainly composed of polyimideor an inorganic insulating layer mainly composed of a Si compound. 5.The piezoelectric device as claimed in claim 2 produced by a methodcomprising: step (A) of forming an unpatterned insulating layer on asubstrate having the lower electrode and the piezoelectric materialformed thereon; step (B) of forming a resist mask on the insulatinglayer at an area where the upper electrode is not to be formed; and step(C) of partially removing the insulating layer from an area where theupper electrode is to be formed to leave the insulating layer at an edgeportion of the area where the upper electrode is to be formed, the leftinsulating layer having a thickness that gradually increases along thedirection from the central area toward the edge surface of the upperelectrode, thereby forming the sloped-thickness insulating layer.
 6. Thepiezoelectric device as claimed in claim 5, wherein the step (C)comprises partially removing the insulating layer from the area wherethe upper electrode is to be formed through a dry etching process. 7.The piezoelectric device as claimed in claim 1, wherein the edge portionof the upper electrode comprises a gradient composition structure wherean insulation property of the upper electrode gradually becomes higheralong the direction from the central area toward the edge surface of theupper electrode.
 8. The piezoelectric device as claimed in claim 7,wherein a main portion other than the edge portion of the upperelectrode is mainly composed of a conductive metal, and the edge portionof the upper electrode comprises a gradient composition structure wherea metal oxide content gradually increases along the direction from thecentral area toward the edge surface of the upper electrode.
 9. Thepiezoelectric device as claimed in claim 8 produced by a methodcomprising: step (D) of forming the upper electrode having a uniformcomposition mainly composed of a conductive metal on a substrate havingthe lower electrode and the piezoelectric material formed thereon; step(E) of forming an oxygen permeating film on the upper electrode, theoxygen permeating film having such oxygen permeability that an area ofthe oxygen permeating film corresponding to the main portion other thanthe edge portion of the upper electrode has a uniform oxygentransmission rate, and an area of the oxygen permeating filmcorresponding to the edge portion of the upper electrode has an oxygentransmission rate that gradually increases along the direction from thecentral area toward the edge surface of the upper electrode; and step(F) of applying oxidation to the upper electrode covered with the oxygenpermeating film to provide the edge portion of the upper electrode withthe gradient composition structure.
 10. The piezoelectric device asclaimed in claim 9, wherein the step (E) comprises forming, as theoxygen permeating film, an oxygen permeating film having asloped-thickness structure where an area of the oxygen permeating filmcorresponding to the main portion other than the edge portion of theupper electrode has a uniform thickness, and an area of the oxygenpermeating film corresponding to the edge portion of the upper electrodehas a thickness that gradually decreases along the direction from thecentral area toward the edge surface of the upper electrode.
 11. Thepiezoelectric device as claimed in claim 10, wherein the oxygenpermeating film comprises a film having an oxygen permeabilitycoefficient at a temperature of 40° C. of not less than 1.0×10⁻¹¹(cm³(STP)·cm/cm²·s·cmHg).
 12. The piezoelectric device as claimed inclaim 8 produced by a method comprising: step (D) of forming the upperelectrode having a uniform composition mainly composed of a conductivemetal on a substrate having the lower electrode and the piezoelectricmaterial formed thereon; step (G) of forming a covering film on theupper electrode; and step (H) of applying oxidation to the upperelectrode covered with the covering film from an edge surface side ofthe upper electrode to provide the edge portion of the upper electrodewith the gradient composition structure.
 13. A method for producing apiezoelectric device, the method producing the piezoelectric device ofclaim 2, the method comprising: step (A) of forming an unpatternedinsulating layer on a substrate having the lower electrode and thepiezoelectric material formed thereon; step (B) of forming a resist maskon the insulating layer at an area where the upper electrode is not tobe formed; and step (C) of partially removing the insulating layer froman area where the upper electrode is to be formed to leave theinsulating layer at an edge portion of the area where the upperelectrode is to be formed, the left insulating layer having a thicknessthat gradually increases along the direction from the central areatoward the edge surface of the upper electrode, thereby forming thesloped-thickness insulating layer.
 14. The method for producing apiezoelectric device as claimed in claim 13, wherein the step (C)comprises partially removing the insulating layer from the area wherethe upper electrode is to be formed through a dry etching process.
 15. Amethod for producing a piezoelectric device, the method producing thepiezoelectric device of claim 8, the method comprising: step (D) offorming the upper electrode having a uniform composition mainly composedof a conductive metal on a substrate having the lower electrode and thepiezoelectric material formed thereon; step (E) of forming an oxygenpermeating film on the upper electrode, the oxygen permeating filmhaving such oxygen permeability that an area of the oxygen permeatingfilm corresponding to the main portion other than the edge portion ofthe upper electrode has a uniform oxygen transmission rate, and an areaof the oxygen permeating film corresponding to the edge portion of theupper electrode has an oxygen transmission rate that gradually increasesalong the direction from the central area toward the edge surface of theupper electrode; and step (F) of applying oxidation to the upperelectrode covered with the oxygen permeating film to provide the edgeportion of the upper electrode with the gradient composition structure.16. The method for producing a piezoelectric device as claimed in claim15, wherein the step (E) comprises forming, as the oxygen permeatingfilm, an oxygen permeating film having a sloped-thickness structurewhere an area of the oxygen permeating film corresponding to the mainportion other than the edge portion of the upper electrode has a uniformthickness, and an area of the oxygen permeating film corresponding tothe edge portion of the upper electrode has a thickness that graduallydecreases along the direction from the central area toward the edgesurface of the upper electrode.
 17. The method for producing apiezoelectric device as claimed in claim 16, wherein the step (E)comprises forming, as the oxygen permeating film, a film having anoxygen permeability coefficient at a temperature of 40° C. of not lessthan 1.0×10⁻¹¹ (cm³(STP)·cm/cm²·s·cmHg).
 18. A method for producing apiezoelectric device, the method producing the piezoelectric device ofclaim 8, the method comprising: step (D) of forming the upper electrodehaving a uniform composition mainly composed of a conductive metal on asubstrate having the lower electrode and the piezoelectric materialformed thereon; step (G) of forming a covering film on the upperelectrode; and step (H) of applying oxidation to the upper electrodecovered with the covering film from an edge surface side of the upperelectrode to provide the edge portion of the upper electrode with thegradient composition structure.
 19. A liquid discharge devicecomprising: the piezoelectric device as claimed in claim 1; and a liquiddischarge member disposed adjacent to the piezoelectric device, theliquid discharge member comprising a liquid reservoir for storing aliquid, and a liquid discharge port for discharging the liquid from theliquid reservoir to the outside in response to application of theelectric field to the piezoelectric material.